Flexible electrical circuits are presently being used commercially in computers, cameras, telephone and PBX systems, medical equipment controls and in automobile radios. These reliable interconnection devices save space and assembly time and it is anticipated that additional uses in aerospace and other applications will be forthcoming now that electronic designers no longer regard flexible circuits as experimental but accept such circuitry as reliable. One problem that has limited their use is the ability of present flexible circuits to remain reliable under extreme temperature variations. This unreliability is due to the low thermal stability of the adhesives presently used in flexible circuit laminates. There is a definite need in the aerospace industry for reliable flexible electrical circuitry for utilizing adhesives of high thermal stability for severe temperature and pressure environments such as may be encountered in applications on missiles; aircraft and spacecraft.
Linear aromatic polyimides derived from polyamic acid precursors are excellent candidates as adhesives for such applications because they are thermally stable at temperatures between 477 K. (400.degree. F.) and 589 K. (600.degree. F.), flexible, and light in weight.
A problem exists, however, in using such derived adhesives to bond polyimide film to itself or to metal surfaces, such as is desired for various circuitry applications, due to the evolution of unwanted volatiles. Such volatiles can be the result of condensation reaction by-products and/or excess solvent, both of which cause difficulties in the preparation of large-area, void-free laminates. Unless features such as porous or permeable films are used (see Bratton, U.S. Pat. No. 3,449,193), sucn volatiles lead to the formation of voids and unlaminated areas in the material.
A process for laminating large areas of void-free high temperature polyimide film is needed for aerospace applications. Several years ago, a particular need for a flexible, high temperature adhesive to bond ultra-thin polyimide film was presented by the proposed NASA Solar Sail Program. A flexible adhesive was needed for joining strips of 0.002 .mu.m (0.08 mil) DuPont polyimide Kapton.RTM. H Film at intervals across each sail blade measuring 8 meters by 7350 meters in length. Kapton.RTM. is available in various thicknesses and is a trademark of the DuPont Company for a linear polyimide film formed from pyromellitic dianhydride (PMDA) and 4,4'-oxydianiline (ODA). Several linear polyimide adhesives were developed for this application using the following monomers: ##STR1##
The adhesives identified as LARC-2, 3, and 4, and formulated as shown in Table I, were prepared in a nontoxic ether solvent and proved successful in bonding 1/4-inch overlaps of thin polyimide Kapton.RTM. film. The adhesives themselves showed good thermal stability when subjected to temperatures of 575 K. in excess of 500 hours and the adhesive bonds also showed excellent retention of strength after aging 6000 hours at elevated temperature. Although these 1/4-inch polyimide bonds were not entirely "void-free", the adhesives were far stronger than the films they joined. A more complete description of these adhesives is found in the January 1979 issue of Adhesive Age, pp. 35-39, which is incorporated herein by reference.
TABLE I ______________________________________ KAPTON .RTM. ADHESIVES Amic Acid T.sub.g Resin Formulation n.sub.inh K (.degree.F.) ______________________________________ LARC-2 BTDA 0.70 520 3,3'-DABP (477) LARC-3 2 BTDA/1 PMDA 0.63 542 3,3'-DABP (516) LARC-4 3 BTDA/1 PMDA 0.50 570 4,4'-DABP (567) ______________________________________
The 1/4-inch overlap of the polyimide bonds described above is small enough in area to provide escape for a large portion of volatiles. A definite need still exists for a method for joining larger areas of polyimide film, so as to produce 100% void-free laminates.
Void-free laminates of thin polyimide film are deemed useful in high temperature packaging or encapsulating for the purpose of environmental protection. The "through-the-thickness" (TTT) strength of thick commercial polyimide film, Kapton.RTM. for example, is very poor and prohibits the use thereof in certain applications. However, by use of the present invention, thick laminates of the commercially thin film can be produced without decreasing the individual layer (TTT) strength and thereby yield a thick polyimide laminate having improved physical property characteristics. It is also anticipated that polyimide film and laminates thereof constructed according to the present invention will prove useful as coatings wherein, the polyimide film is primed with an adhesive in accordance with this invention, and the film bonded to different substrates to serve as a protective high temperature resistant coating thereon.
It is therefore an object of the present invention to provide an improved process for bonding high temperature polyimide film.
Another object of the present invention is a method for preparing flexible, large-area, 100% void-free laminates from polyimide film.
Another object of the present invention is an improved method for laminating polyimide film to itself and/or to metal surfaces for ultimate use in flexible circuit applications.
An additional object of the present invention is the use of a thermoplastic bonding process to make laminates free of solvents and voids to obtain high temperature resistant polyimide laminate structures.
Yet another object of the present invention is to provide void-free polyimide and polyimide-metal laminates.